1. Field of the Invention
The invention relates to optical fiber fabrication, in particular the disposal of by-products of such fabrication.
2. Discussion of the Related Art
Glass optical fiber is typically drawn from a solid preform containing an overcladding that surrounds an inner cladding and core. An overcladding tube is generally formed separately from the inner cladding and core, and the components are then brought together to make the preform. The overcladding does not have to meet the demanding purity and uniformity specifications of the core and inner cladding, and some efforts at improving manufacturing efficiency and lowering cost of optical fiber manufacturing processes have therefore focused on the overcladding. These efforts have led to the use of sol-gel processes to form overcladding tubes.
U.S. Pat. No. 5,240,488, the disclosure of which is hereby incorporated by reference, discloses a sol-gel process for production of overcladding tubes. In the process, an aqueous colloidal silica dispersion is used. The dispersion is typically stabilized by addition of a base such as tetramethylammonium hydroxide (TMAH). It is also possible to use tetraethylammonium hydroxide (TEAH). TMAH and TEAH are believed to stabilize silica particles by the following mechanism: Introduction of the TMAH or TEAH solution into a silica dispersion raises the pH value. The silica then takes on a negative surface charge due to ionization of silanol groups present on the surface, in accordance with the following reaction: EQU --Si--OH+OH.sup.- .fwdarw.--Si--O.sup.- +H.sub.2 O.
The negative charge on the silica particles creates mutual repulsion, preventing substantial agglomeration and maintaining the stability of the dispersion. At a later stage in the process, as discussed at Col. 15, lines 39-65 of the '488 patent, a gelling agent such as methyl formate is added to reduce the pH. The methyl formate, through reaction with water and/or base, neutralizes the negatively-charged silica to a degree where gelation is induced.
While processes such as that of the '488 patent produce good results, use of TMAH has caused some inconveniences in large-scale optical fiber fabrication. As shown in the Table in Cols. 11 and 12 of the '488 patent, once a gelled tube is dried, the tube must be heated to drive off volatile organic materials and water. During this heating, the tetramethylammonium salt that is present (e.g., tetramethyl ammonium formate in the reaction of the '488 patent) breaks down to an extent into trimethylamine (TRIMA), which has an undesirable odor, and is volatile and flammable. (TRIEA formed when using TEAH exhibits similar, undesirable properties.) The TRIMA or TRIEA is therefore burned or trapped and subsequently removed. Neither option is desirable, and both require significant off-line treatment that affects the productivity and expense of the overall fabrication process. Additionally, burning is expensive, and may lead to emission of undesirable gases that are removed by expensive emission controls.
Thus, the use of TMAH (or TEAH) in sol-gel overcladding fabrication has the potential to complicate an optical fiber manufacturing process. An easier, less-expensive method for disposing of TRIMA (or TRIEA) is desirable.